In the prior art, there are different requirements for thermal loads of the combustor of a gas water heater or a wall-hanging boiler according to different demands for the amount and temperature of hot water. For example, when there is a need for a large amount of hot water, the combustor needs to have a larger thermal load, and when a small amount of hot water is desired, the combustor may only have a smaller thermal load.
Currently, the thermal load of a combustor is controlled mainly by controlling currents of a proportional valve and a fan. To be specific, when a larger thermal load is needed, a larger current will be supplied to the proportional valve, so that the proportional valve can have a larger opening, thereby more fuel gas will be allowed to pass through the proportional valve and reach the combustor for combustion; meanwhile, a larger current will also be supplied to the fan to provide it with a larger rotation speed to increase the flow of combustion air, so that the fuel gas can be better combusted in the combustor, and thereby the combustor will have a larger thermal load.
Under ideal conditions, the currents of the proportional valve and the fan are in correspondence relationship with each other, i.e., a determined current allows the proportional valve to have a determined opening. In general, the flow of fuel gas that passes through the proportional valve is in correspondence relationship with the opening of the proportional valve, and, since the flow of fuel gas is also in correspondence relationship with the flow of combustion air required for combustion, the current of the proportional valve and the flow of combustion air are also in correspondence relationship with each other. Furthermore, the flow of combustion air is formed in correspondence relationship with both of the demanded rotation speed and current of the fan, so that the current of the proportional valve and the current of the fan are also in correspondence relationship with each other. Due to the above correspondence relationships, the gas water heater and wall-hanging boiler in the prior art mostly apply a method of controlling thermal loads of the combustor by correspondingly controlling the currents of the proportional valve and the fan.
However, in real life, the operation environments for most gas water heaters or wall-hanging boilers are not ideal. In a case where there is wind in the operating environment, a reverse wind pressure may be generated at the exhaust channel of the gas water heater or wall-hanging boiler, blocking the exhaust of the gas water heater or wall-hanging boiler. When a reverse wind pressure occurs, the rotational resistance of the fan is increased, so that the current of the fan is decreased. At this point, this may lead to a reduction of the flow of combustion air, causing deterioration of the combustion condition and even flameout. In order to prevent the above situations from happening, a current compensation mechanism is provided for the fan, which will compensate the current of the fan when the current of the fan is decreased, so as to recover the rotational speed of the fan. Please further refer to FIG. 1. The existing compensation mechanisms mostly employ a method of sectional compensating the current of the fan. For example, when the reduction of current of the fan is less than 7%, no compensation or rotational speed increasing is performed for the current of the fan; when the reduction of current of the fan is 7%-13%, the fan is compensated by increasing its rotational speed to 500 rpm; when the reduction of current of the fan is 13%-25%, the fan is compensated by increasing the rotational speed to 700 rpm; and when the reduction of current of the fan is larger than 25%, a failure is reported. As such, when the reduction of current is smaller than a threshold value, no compensation will be performed for the current of the fan, at this point, the flow of combustion air is reduced, which influences the combustion condition and thereby reduces the thermal load of the combustor. Besides, due to the existence of the reverse wind pressure, even if the rotational speed of the fan is increased, the matching of the flow of combustion air is still inaccurate, and the flow of combustion air is still smaller than that in a state free of reverse wind pressure. It can be seen from the above that after the rotational speed of the fan is compensated, since the flow of combustion air is relatively small, the thermal load of the water heater is still low and is hard to satisfy the demands for the amount and temperature of hot water.